There are a number of applications requiring the detection of and information on an electrostatic field, as for example in a plant where two materials are rubbing against one another. Attempts have heretofore been made to provide apparatus suitable for this purpose but in general the principal areas of deficiency of such apparatus include insensitivity, instability, inaccuracy, and a lack of sufficient measurement information.
The most commonly known apparatus for measuring electrostatic fields include the use of an ion drift chamber and the use of a mechanical modulator. Both of these prior known approaches have uncertainty problems, but it has been found that the mechanical modulator type can be controlled to give reasonably good accuracy and stability.
The apparatus of the present invention in general is in the category of a mechanical modulator. All mechanical modulator-type apparatus for detecting electrostatic fields has the electrostatic field being measured passed through a slotted disc or rotary chopper blade and this field is interrupted as ground segments and open segments alternately pass in front of one or more sensing plates. The one or more sensing plates which are mounted behind the disc or chopper produce an electric signal output when the electrostatic field falls on them and a reverse electric output signal when the field is interrupted by the disc or chopper. The intensity of the field is related to the amplitude of the output signal and the polarity of the field is indicated by the phase of the output signal.
The output signal is symmetrical and must be phase demodulated to obtain information about the strength polarity and potential of the electrostatic field being measured. Phase demodulation requires careful timing and therefore an extra detector is added to determine the position of the rotary blade in its rotation. In nearly all of the mechanical modulator-type apparatus a capacitor coupling is used between the charge to be detected and the detector, and through some mechanical means this coupling is modulated.
In these prior known types of apparatus the impedance between the case and ground is of very unpredictable magnitude and the effect of this impedance must be eliminated if accuracy is to be achieved. This has been done in the past by using a differential sensor arrangement.
Accordingly, it is an object of the present invention to provide a novel and improved comparatively simple apparatus for detecting and measuring electrostatic fields.
Another object of this invention is to provide a novel apparatus for measuring electrostatic fields characterized by accuracy, stability and the development of useful information regarding the polarity of the electrostatic field.
Still another object of this invention is to provide a novel measuring apparatus for electrostatic fields which does not have the effects of stray capacitance between the case and ground, and does not require synchronous rectification to obtain polarity and intensity data about the field.
Yet a further object of this invention is to provide a novel and improved apparatus for detecting and measuring electrostatic fields characterized by the use of at least two sensing plates, each of which forms one plate of a capacitor with the surface area producing the field being measured, and a rotary field-interrupting blade whereby the plate area, and thereby the capacitances of the two capacitors, vary inversely over a full cycle of operation with a reversal in each half-cycle to provide an instantaneous current in the plate for producing a non-symmetrical output signal that is a measure of the polarity and intensity of the electrostatic field exposed to the sensing plates.